Welded joint and method of manufacturing welded joint

ABSTRACT

A welded joint having high strength and good hydrogen embrittlement resistance is provided. A welded joint is a welded joint obtained by welding a base material using a welding material. The base material has a chemical composition of, in mass %: C: 0.005 to 0.1%; Si: up to 1.2%; Mn: 2.5 to 6.5%; Ni: 8 to 15%; Cr: 19 to 25%; Mo: 0.01 to 4.5%; V: 0.01 to 0.5%; Nb: 0.01 to 0.5% Al: less than 0.05%; N: 0.15 to 0.45%; O: up to 0.02%; P: up to 0.05%; and S: up to 0.04%, and a balance being iron and impurities, and which satisfies Equation (1). The welding material has a chemical composition which satisfies Equations (1) and (2).
 
Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C≥29  (1)
 
0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo≤−1.0  (2)

TECHNICAL FIELD

The present invention relates to a welded joint and a method ofmanufacturing a welded joint, and more particularly to anaustenitic-steel welded joint and a method of manufacturing anaustenitic-steel welded joint.

BACKGROUND ART

In recent years, research has been done to commercialize transportmachines that use hydrogen, natural gas or the like as its drivingenergy. Such commercialization requires establishment of a utilizationenvironment where such gases under high pressure can be stored andtransported. At the same times, high-strength materials used in theseapplications that have tensile strengths higher than 800 MPa have beendeveloped and their applications have been considered.

WO 2004/083476, WO 2004/083477 and WO 2004/110695 propose increasing Mnto increase the solubility of N and including V or including acombination of V and Nb to take advantage of solute strengthening of Nand precipitation strengthening of nitride to provide an austeniticstainless steel with increased strength.

When a high-strength austenitic steel is used as a structure, parts madetherefrom must be welded together. To provide sufficient performance inuse, the welded portion is required to have a strength substantiallyequal to that of the base material. WO 2004/110695, JP Hei5(1993)-192785and JP 2010-227949 propose actively utilizing Al, Ti and Nb to provide awelding material and weld metal having a tensile strength exceeding 800MPa.

These welding materials and weld metals provided by using these weldingmaterials must be subjected to heat treatment after welding in order toprovide high strength. A prolonged heat treatment after welding means alimitation in manufacturing and may cause an increase in manufacturingcosts.

WO 2013/005570 proposes taking advantage of solute strengthening ofwelded metal by N to provide an austenitic-steel welded joint havinghigh strength and good hydrogen embrittlement resistance withoutperforming heat treatment after welding.

DISCLOSURE OF THE INVENTION

In making the austenitic-steel welded joint of WO 2013/005570, a weldingmaterial containing 0.15 to 0.35% N is used for welding to cause theweld metal to contain 0.15 to 0.35% N. Because of that, weldingmaterials are limited to be used for this austenitic-steel welded joint.This austenitic-steel welded joint requires the use of a weldingmaterial containing a large amount of N and thus cannot be efficientlyproduced, and, under some welding conditions, may have weld defects suchas blowholes.

Further, even when a welding material containing a large amount of N isused, N may be separated from the weld metal during welding. To takeadvantage of solute strengthening by N, N must remain in the weld metal.As long as a conventional welded joint is to be produced, it isdifficult to provide a certain N content in the weld metal in a stablemanner under a wide range of welding conditions.

Further, a welded joint to be used in handling high-pressure hydrogen isrequired to have good hydrogen embrittlement resistance.

An object of the present invention is to provide a welded joint havinghigh strength and good hydrogen embrittlement resistance.

A welded joint according to the present invention is a welded jointobtained by welding a base material using a welding material. The basematerial has a chemical composition of, in mass %: C: 0.005 to 0.1%; Si:up to 1.2%; Mn: 2.5 to 6.5%; Ni: 8 to 15%; Cr: 19 to 25%; Mo: 0.01 to4.5%; V: 0.01 to 0.5%; Nb: 0.01 to 0.5%; Al: less than 0.05%; N: 0.15 to0.45%; O: up to 0.02%; P: up to 0.05%; and S: up to 0.04%, and a balancebeing iron and impurities. The welding material has a chemicalcomposition of, in mass %: C: 0.005 to 0.1%; Si: up to 0.7%; Mn: 0.5 to3%; Ni: 8 to 23%; Cr: 17 to 25%; Mo: 0.01 to 4%; V: 0 to 0.5%; Nb: 0 to0.5%; Al: less than 0.05%; N: less than 0.15%; O: up to 0.02% P: up to0.03%; and S: up to 0.02%, and a balance being iron and impurities. Thechemical composition of the base material satisfies Equation (1). Thechemical composition of the welding material satisfies Equations (1) and(2).Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C≥29  (1)0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo≤−1.0  (2)

Here, the contents of elements (in mass %) are substituted for thesymbols of these elements in Equations (3) and (2).

The present invention provides a welded joint having high strength andgood hydrogen embrittlement resistance.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present inventors did research to find conditions under which awelded joint can be provided having high strength and good hydrogenembrittlement resistance without performing heat treatment after weldingand without using a welding material containing a large amount of N.They found out the following points (a) to (c).

(a) When the austenitic phase of the weld metal is unstable, its weldresidual strain and the subsequent treatment transform the austeniticphase in the weld metal into martensite. This decreases the hydrogenembrittlement resistance of the weld metal. In view of this, adjustingthe chemical composition of the weld metal to stabilize the austeniticphase will improve the hydrogen embrittlement resistance of the weldmetal. More specifically, the weld metal suitably satisfies Equation (1)below. The chemical composition of the weld metal satisfies Equation (1)if the chemical compositions of both the base material and weldingmaterial satisfy Equation (1).Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C≥29  (1)

Here, the contents of elements (in mass %) are substituted for thesymbols of these elements in Equation (1).

(b) To provide a welded joint having a strength substantially equal tothat of the base material, it is effective to dissolve a large amount ofN in the weld metal for solute strengthening by N. This is achieved ifthe chemical composition of the welding material satisfies Equation (2).If the chemical composition of the welding material satisfies Equation(2), it is possible to dissolve a large amount of N in the weld metaleven if the N content in the welding material is lower than 0.15 mass %.0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo≤−1.0  (2)

Here, the contents of elements (in mass %) are substituted for thesymbols of these elements in Equation (2).

(c) A still higher tensile strength can be achieved if the height ofexcess weld metal formed on the outer surface of the welded joint (i.e.surface excess weld metal height) is adjusted depending on the chemicalcomposition of the welding material. More specifically, the surfaceexcess weld metal height h (mm) suitably satisfies Equation (3).1.9×(0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo)+3≤h  (3)

Here, the contents of the elements in the welding material (in mass %)are substituted for the symbols of these elements in Equation (3).

The welded joint of the present invention was made based on the abovefindings. The welded joint in an embodiment of the present inventionwill be described below in detail. In the description below, “%” for thecontent of an element means mass percentage.

The welded joint in the present embodiment is obtained by welding a basematerial with a welding material. The welded joint includes a basematerial and a weld metal. The weld metal is formed by a part of thebase material and the welding material melting together and solidifying.The welded joint may be, for example, steel pipes or steel plates withtheir adjacent ends/edges welded together.

[Chemical Composition]

The base material and welding material include the chemical compositionas described below.

C: 0.005 to 0.1% (Base Material and Welding Material)

Carbon (C) stabilizes austenite. On the other hand, if an excessiveamount of C is contained, heat during welding may cause carbide alonggrain boundaries, which decreases corrosion resistance and toughness. Inview of this, for both the base material and welding material, the Ccontent should be in the range from 0.005 to 0.1%. The preferable lowerlimit of the C content is 0.008%. The preferable upper limit of the Ccontent is 0.08%.

Si: Up to 1.2% (Base Material), Up to 0.7% (Welding Material)

Silicon (Si) deoxidize steel. Si also improves the corrosion resistanceof steel. However, if an excessive amount of Si is contained, thetoughness of the steel decreases. In view of this, the Si content in thebase material should be up to 1.2%. The preferable upper limit of the Sicontent in the base material is 1.0%.

In addition, if an excessive amount of Si is contained in a weld metalformed by the welding material melting, Si segregates along columnarcrystal boundaries when solidifying, which decreases the melting pointof the liquid phase, increasing solidification crack sensitivity. Inview of this, the upper limit of the Si content in the welding materialshould be lower than that for the base material. Thus, the Si content inthe welding material should be up to 0.7%. The preferable upper limit ofthe Si content in the welding material is 0.6%. No lower limit is neededfor the Si content; however, an excessively low amount may result ininsufficient deoxidization, which increases the index of cleanliness ofthe steel and deteriorates the cleanliness, increasing costs. Thus, thedesirable lower limit of Si is 0.01% for both the base material andwelding material.

Mn: 2.5 to 6.5% (Base Material), 0.5 to 3% (Welding Material)

Manganese (Mn) deoxidizes steel. Mn also stabilizes an austenitic phase.Mn further increases the solubility of N in the weld metal duringproduction of the base material and during welding, thereby indirectlycontributing to increasing the strength of the weld metal. On the otherhand, if an excessive amount of Mn is contained, the ductility of thesteel decreases. In view of this, the Mn content in the base materialshould be in the range from 2.5 to 6.5%. The preferable lower limit ofthe Mn content in the base material is 2.7%. The preferable upper limitof the Mn content in the base material is 6%.

The solidification rate for a weld metal formed by the welding materialmelting is higher than that for the base material being produced, andthus the reduction in N during solidification is smaller. In view ofthis, the lower limit of the Mn content in the welding material may belower than that for the base material. On the other hand, in the case ofthe welding material, a reduction in ductility may make it difficult toproduce fine wires therefrom. In view of this, the upper limit of the Mncontent in the welding material should be lower than that for the basematerial. Thus, the Mn content in the welding material should be in therange from 0.5 to 3%. The preferable lower limit of the Mn content inthe welding material is 0.7%. The preferable upper limit of the Mncontent in the welding material is 2.5%.

Ni: 8 to 15% (Base Material), 8 to 23% (Welding Material)

Nickel (Ni) stabilizes an austenitic phase. To achieve this effect in astable manner, 8% or more Ni must be contained. However, an excessiveamount of Ni decreases the solubility of N in the weld metal duringproduction of the base material. Further, since Ni is an expensiveelement, an excessive content means increased costs. In view of this,the upper limit of the Ni content in the base material should be 15%.Further, the preferable lower limit of the Ni content in the basematerial is 9%. The preferable upper limit of the Ni content in the basematerial is 14.5%.

In the weld metal, too, Ni stabilizes the austenitic phase. To achievethis effect in a stable manner, 8% or more Ni must be contained in thewelding material. However, an excessive content of Ni decreases thesolubility of N in the weld metal. Further, since Ni is an expensiveelement, an excessive content means increased costs even for weldingmaterials in petty manufacturing. In view of this, the upper limit of Niin the welding material should be 23%. The preferable lower limit of theNi content in the welding material is 9%. The preferable upper limit ofthe Ni content in the welding material is 22.5%.

Cr: 19 to 25% (Base Material), 17 to 25% (Welding Material)

Chromium (Cr) increases the corrosion resistance of steel. Cr furtherincreases the solubility of N in the weld metal during production of thebase material and during welding, thereby indirectly contributing toincreasing the strength of the weld metal. On the other hand, if anexcessive amount of Cr is contained, a large amount of coarse particlesof a carbide such as M₂₃C₆ which decreases ductility and toughness maybe produced. Further, if an excessive amount of Cr is contained, thesteel may be made brittle in some types of weld gas environment. In viewof this, the Cr content in the base material should be in the range from19 to 25%. The preferable lower limit of the Cr content in the basematerial is 19.2%. The preferable upper limit of the Cr content in thebase material is 24.5%.

The solidification rate for a weld metal formed by the welding materialmelting is higher than that for the base material being produced, andthus the reduction in N during solidification is smaller. In view ofthis, the lower limit of the Cr content in the welding material may belower than that for the base material. Thus, the Cr content in thewelding material should be in the range from 17 to 25%. The preferablelower limit of the Cr content in the welding material is 18.2%. Thepreferable upper limit of the Cr content in the welding material is24.5%.

Mo: 0.01 to 4.5% (Base Material), 0.01 to 4% (Welding Material)

Molybdenum (Mo) dissolves in a matrix or precipitates in the form of acarbonitride, increasing the strength of the steel. Mo also increasesthe corrosion resistance of the steel. On the other hand, if anexcessive amount of Mo is contained, this increases costs. Also, when anexcessive amount of Mo is added to the steel, the steel is saturated interms of Mo's effects. In view of this, the Mo content in the basematerial should be in the range from 0.01 to 4.5%. The preferable lowerlimit of the Mo content in the base material is 0.03%. The preferableupper limit of the Mo content in the base material is 4%.

The solidification rate for a weld metal formed by the welding materialmelting is higher than that for the base material being produced, andthus the reduction in N during solidification is smaller. In view ofthis, the upper limit of the Mo content in the welding material shouldbe lower than that for the base material. Thus, the Mo content in thewelding material should be in the range from 0.01 to 4%. The preferablelower limit of the Mo content in the welding material is 0.03%. Thepreferable upper limit of the Mo content in the welding material is3.8%.

V: 0.01 to 0.5% (Base Material), 0 to 0.5% (Welding Material)

Vanadium (V) dissolves in a matrix or precipitates in the form of acarbide, increasing the strength of the steel. On the other hand, if anexcessive amount of V is contained, a large amount of carbideprecipitates, decreasing the ductility of the steel. In view of this,the V content in the base material should be in the range from 0.01 to0.5%. The preferable upper limit of the V content in the base materialis 0.4%.

Vanadium (V) does not need to be added to the welding material. That is,V is an optional element for the welding material. If the weldingmaterial contains V, this increases the strength of the weld metal. Inview of this, the V content in the welding material should be in therange from 0 to 0.5%. If V is added, the preferable lower limit of the Vcontent in the welding material is 0.01%. The preferable upper limit ofthe V content in the welding material is 0.4%.

Nb: 0.01 to 0.5% (Base Material), 0 to 0.5% (Welding Material)

Niobium (Nb) dissolves in a matrix or precipitates in the form of acarbonitride, increasing the strength of the steel. On the other hand,if an excessive amount of Nb is contained, a large amount ofcarbonitride precipitates, decreasing the ductility of the steel. Inview of this, the Nb content in the base material should be in the rangefrom 0.01 to 0.5%. The preferable upper limit of the Nb content in thebase material is 0.4%.

Niobium (Nb) does not need to be added to the welding material. That is,Nb is an optional element for the welding material. If the weldingmaterial contains Nb, this increases the strength of the weld metal. Inview of this, the Nb content in the welding material should be in therange from 0 to 0.5%. If Nb is added, the preferable lower limit of theNb content in the welding material is 0.01%. The preferable upper limitof the Nb content in the welding material is 0.4%.

Al: Less than 0.05% (Base Material and Welding Material)

Aluminum (Al) deoxidizes steel. On the other hand, if an excessiveamount of Al is contained, a large amount of nitride precipitates,decreasing the ductility of the steel. In view of this, for both thebase material and welding material, the Al content should be less than0.05%. The preferable upper limit of the Al content is 0.04%. The lowerthe Al content, the better. However, an excessively low amount of Alresults in insufficient deoxidization. Further, an excessively lowamount of Al increases the index of cleanliness of the steel. Further,an excessively low amount of Al means increased costs. In view of this,the preferable lower limit of the Al content is 0.0001%.

N: 0.15 to 0.45% (Base Material), Less than 0.15 (Welding Material)

N dissolves in a matrix or forms fine nitride particles, increasing thestrength of the steel. On the other hand, if an excessive amount of N iscontained, the hot workability of the steel decreases. In view of this,the N content in the base material should be in the range from 0.15 to0.45%. The preferable lower limit of the N content in the base materialis 0.16%. The preferable upper limit of the N content in the basematerial is 0.42%.

In a weld metal formed by the welding material, melting, an excessiveamount of N cannot melt in the molten pool during welding, which mayresult in blowholes and/or pits. In view of this, the N content in thewelding material should be less than 0.15%. The preferable lower limitof the N content in the welding material is 0.01%. The preferable upperlimit of the N content in the welding material is 0.13%.

The balance in the chemical composition of each of the base material andweld metal is Fe and impurities. Impurity means an element originatingfrom ore or scraps used as the raw material of steel or an element thathas entered for various reasons during the manufacturing process. In thepresent embodiment, the contents of the impurities O, P and S arelimited to the ranges described below.

O: 0.02% (Base Material and Welding Material)

Oxygen (O) is an impurity. If an excessive amount of O is contained, hotworkability during production of the base material and welding materialdecreases. Further, if an excessive amount of O is contained, thetoughness and ductility of the weld metal decreases. In view of this,for both the base material and welding material, the O content should beup to 0.02%. The preferable upper limit of the O content is 0.01%.

P: Up to 0.05% (Base Material), Up to 0.03% (Welding Material)

Phosphorus (P) is an impurity. If an excessive amount of P is contained,hot workability during production of the base material and weldingmaterial decreases. In view of this, the P content in the base materialshould be up to 0.05%. The preferable upper limit of the P content inthe base material is 0.03%.

In a weld metal formed by the welding material melting, P decreases themelting point of the liquid phase when solidifying, increasing thesolidification crack sensitivity of the weld metal. In view of this, theupper limit of the P content in the welding material should be lowerthan that for the base material. Thus, the P content in the weldingmaterial is up to 0.03%. The preferable upper limit of the P content inthe welding material is 0.02%.

S: Up to 0.04% (Base Material), 0.02% (Welding Material)

Sulfur (S) is an impurity. If an excessive amount of S is contained, hotworkability during production of the base material and welding materialdecreases. In view of this, the S content in the base material should beup to 0.04%. The preferable upper limit of the S content in the basematerial is 0.03%.

In a weld metal formed by the welding material melting, S decreases themelting point of the liquid phase when solidifying, increasing thesolidification crack sensitivity of the weld metal. In view of this, theupper limit of the S content in the welding material should be lowerthan that for the base material. Thus, the S content in the weldingmaterial should be up to 0.02%. The preferable upper limit of the Scontent in the welding material is 0.01%.

Further, the chemical compositions of the base material and weldingmaterial in the present embodiment satisfy Equation (1) provided below.Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C≥29  (1)

Here, the contents of elements (in mass %) are substituted for thesymbols of these elements in Equation (1).

If the austenitic phase is stable in a hydrogen environment, goodhydrogen embrittlement resistance is present. The weld metal is amicrostructure produced by solidification by rapid cooling, and thus theaustenitic phase tends to be unstable. As discussed above, the weldmetal is formed by a part of the base material and the welding materialmelting together and solidifying. If the chemical compositions of boththe base material and welding material satisfy Equation (1), theaustenitic phase is also stable in the weld metal. This increases thehydrogen embrittlement resistance of the welded joint.

The value of the left side of Equation (1) is preferably 32 or higher,and more preferably 34 or higher.

The chemical composition of the welding material in the presentembodiment further satisfies Equation (2) provided below.0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo≤−1.0  (2)

Here, the contents of elements (in mass %) are substituted for thesymbols of these elements in Equation (2).

The welding material melts during welding to form a weld metal. At thismoment, N may be separated from the weld metal. If N is separated fromthe weld metal, the effect of solute strengthening cannot be provided,which decreases the strength of the weld metal. If the chemicalcomposition of the welding material satisfies Equation (2), the activityof N is low, which prevents N from being separated from the weld metal.Thus, a large amount of N can be dissolved in the weld metal even if theN content in the welding material is lower than 0.15%.

The lower the value of the left side of Equation (2), the better. Thelower the value of the left side of Equation (2), the smaller thesurface excess weld metal height, described below, is allowed to be. Thevalue of the left side of Equation (2) is preferably −1.1 or lower, andmore preferably −1.3 or lower.

Preferably, the welded joint in the present embodiment has a surfaceexcess weld metal height h (mm) that satisfies Equation (3) providedbelow.1.9×(0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo)+3≤h  (3)

Here, the contents of the elements in the welding material (in mass %)are substituted for the symbols of these elements in Equation (3).

When the value of the left side of Equation (2) is P2, Equation (3) maybe expressed as provided below. That is, Equation (3) indicates that thesurface excess weld metal height h is adjusted depending on the activityof N in the welding material.1.9×P2+3≤h

Surface excess weld metal height means the distance (mm) between thesurface of the base material and the uppermost position of the weldbead. If the surface excess weld metal height h of the welded jointsatisfies Equation (3), the welded joint has increased tensile strength.More specifically, the welded joint has a tensile strength substantiallyequal to that of the base material.

[Manufacturing Method]

First, an example method of manufacturing the base material will bedescribed. A steel having the chemical composition for the base materialdescribed above is smelted. The smelting may be performed by an electricfurnace, an Ar—O₂ mixture bottom-blown decarburizing furnace (AODfurnace), or a vacuum decarburizing furnace (VOD furnace). The smeltedsteel is used to produce an ingot by the ingot-making method.Alternatively, the smelted steel may be used to produce a slab bycontinuous casting.

The ingot or slab is used to produce a base material. The base materialmay be a steel plate or steel pipe, for example. The steel plate may beproduced by subjecting the ingot or slab to hot working such as hotforging or hot rolling, for example. The steel pipe may be produced by,for example, subjecting the ingot or slab to hot working to produce around billet, and subjecting the round billet to hot working such aspiercing-rolling, hot extrusion or hot forging. Alternatively, the steelpipe may be produced by bending a steel plate to form an open pipe andwelding those edges of the open pipe that extend in the longitudinaldirection.

Heat treatment is performed on the base material. More specifically, Thebase material is placed in a heat treatment furnace and soaked at 1000to 1200° C. Thereafter, as necessary, cold rolling and a secondary heattreatment at 800 to 1200° C. are performed. Thus, a base material havinga tensile strength of 800 MPa can be provided in a stable manner.

Next, an example method of manufacturing the welding material will bedescribed. A steel having the chemical composition for the weldingmaterial described above is smelted. The smelted steel is cast into aningot. The ingot is hot-worked to produce a welding material. Thewelding material may be in the shape of a rod or block.

Heat treatment is performed on the welding material in a similar mannerto that for the base material. Thereafter, as necessary, cold rollingand a secondary heat treatment at 800 to 1250° C. are performed.

The welding material described above is used to weld the base materialdescribed above. This provides a welded joint. The welding method maybe, for example, TIG welding, MIG welding, MAG welding, or submergewelding. During welding, a part of the base material and the weldingmaterial melt together and solidify to form a weld metal.

EXAMPLES

The present invention will be described in more detail using Examples.The present invention is not limited to these Examples.

A steel labeled with character A having the chemical composition shownin Table 1 was melted in a laboratory to produce an ingot. The ingot wassubjected to hot forging, hot rolling and heat treatment to produce asteel pipe (base material) with an outer diameter of 9.53 mm, a platethickness of 2.2 mm and a length of 60 mm.

TABLE 1 Chemical composition (in mass %, balance Fe and impurities)Character C Si Mn P S Ni Cr Mo V Nb Al N O A 0.03 0.40 4.40 0.017 0.00212.0 22.0 2.1 0.20 0.20 <0.001 0.30 0.015

If the chemical composition of the base material is substituted intoEquation (1), the value of the left side of Equation (1) is 34, whichsatisfies Equation (1).

Steels labeled with characters O to Z having the chemical compositionsshown in Table 2 were melted in a laboratory to produce ingots. “-” inTable 2 indicates that the content of the associated element is at animpurity level. The ingots were subjected to hot forging, hot rolling,primary heat treatment, cold treatment and secondary heat treatment toproduce weld wires with an outer diameter of 1.2 mm (welding materials)

TABLE 2 Chemical composition (in mass %, balance Fe and impurities)Character C Si Mn P S Ni Cr Mo V Nb Al N O O 0.098 0.42 2.29 0.001 0.00212.5 24.3 2.79 0.10 0.1 0.004 0.10 0.011 P 0.096 0.42 2.28 0.001 0.00212.5 24.3 2.78 — 0.1 0.003 0.14 0.011 Q 0.097 0.08 2.42 0.001 0.002 12.224.6 2.94 0.23 0.1 0.004 0.10 0.012 R 0.099 0.45 2.34 0.008 0.001 13.524.4 2.22 — — 0.002 0.05 0.006 S 0.019 0.52 1.52 0.020 0.001 20.5 23.12.16 0.10 — 0.001 0.08 0.006 T 0.028 0.20 0.75 0.010 0.001 8.9 20.5 0.01— — 0.002 0.10 0.006 U 0.009 0.46 2.26 0.014 0.001 13.5 19.0 3.84 — —0.003 0.05 0.006  V* 0.008 0.30 1.21 0.001 0.002 12.3 23.6 2.25 — —0.003 0.19 0.007  W* 0.009 0.39 2.02 0.002 0.002 11.1 22.2 2.21 — —0.003 0.33 0.007  X* 0.005 1.10 2.18 0.002 0.002 8.1 26.7 — — — 0.0040.10 0.006  Y* 0.100 0.70 2.25 0.014 0.002 18.8 19.0 3.82 — — 0.004 0.110.006  Z* 0.005 0.23 1.49 0.002 0.002 22.2 17.3 2.15 — — 0.004 0.130.006

Circumferential edge preparation was performed on the steel pipedescribed above and, thereafter, the base material and welding materialswere combined as shown in Table 3 to produce welded joints withdifferent surface excess weld metal heights. The welded joints wereproduced with different welding heat inputs, welding-pass numbers andwelding directions. The rate at which a welding material was fed wasvaried depending on the welding heat input.

TABLE 3 Welding heat Welding- Test Base Welding input Welding passcharacter material material [kJ/cm] direction number J1 A O 15.0-65.0horizontal 1 J2 A O 3.2-8.0 2 J3 A O 3.2-8.0 2 J4 A O 3.2-8.0 2 J5 A P15.0-65.0 1 J6 A P 3.2-8.0 2 J7 A P 3.2-8.0 2 J8 A P 3.2-8.0 2 J9 A Q15.0-65.0 1 J10 A Q 3.2-8.0 2 J11 A Q 3.2-8.0 2 J12 A Q 3.2-8.0 2 J13 AR 42.0-65.0 1 J14 A R 3.2-8.0 2 J15 A R 3.2-8.0 2 J16 A S 42.0-65.0 1J17 A S 3.2-8.0 2 J18 A S 3.2-8.0 2 J19 A T 42.0-65.0 1 J20 A T 3.2-8.02 J21 A T 3.2-8.0 2 J22 A U 42.0-65.0 1 J23 A U 3.2-8.0 2 J24 A U3.2-8.0 2 J25 A V* 15.0-65.0 1 J26 A W* 15.0-65.0 1 J27 A W* 3.2-8.0 2J28 A X* 3.2-8.0 2 J29 A X* 3.2-8.0 2 J30 A X* 3.2-8.0 2 J31 A Y*3.2-8.0 2 J32 A Y* 3.2-8.0 2 J33 A Z* 3.2-8.0 2 J34 A Z* 3.2-8.0 2 J35 AQ 3.2-8.0 vertical 2 J36 A R 42.0-65.0 1 J37 A R 3.2-8.0 2 J38 A W*15.0-65.0 1 J39 A W* 3.2-8.0 2 *indicates deviation from the conditionrequired by the invention

“Horizontal” and “vertical” for welding direction indicate that weldingoccurred in a “flat position” and “vertical position”, respectively, incompliance with JIS Z 3001. More specifically, “horizontal” means thatwelding occurred in a horizontal (i.e. flat) position relative to theground. When welding occurs in a flat position, welding occurs withoutworking against gravity, which generally means the most straightforwardweld position for direction). On the other hand, “vertical” usuallymeans that welding occurs in an upward direction relative to the ground.When welding occurs in a vertical position, welding occurs againstgravity, and thus molten metal may drip, which makes the weldingdifficult and may cause weld defects.

The surface excess weld metal heights of the produced welded joints weremeasured.

From each of the produced welded joints, a test specimen including thewelded portion was extracted. The cut surface of each of the extractedtest specimens was polished and observed by optical microscopy todetermine whether there were weld defects. The welded joints that had noweld defects such as blowholes were determined to be good.

From each of the produced welded joints, two pipe-shaped tensile testspecimens each having weld metal at the center of the parallel portionwere produced and subjected to tensile testing at room temperature. Inthe tensile testing, the welded joints that exhibited a tensile strengthequal to or higher than 800 MPa were determined to be good.

From each of the produced welded joints, pipe-shaped low-strain-ratetensile test specimens each having a parallel portion made of the weldmetal were extracted. The extracted test specimens were subjected tolow-strain-rate tensile testing in the atmosphere and a high-pressurehydrogen environment at 85 MPa. The strain rate was 3×10⁻⁵/s. In thelow-strain-rate tensile testing, the welded joints in which the ratiobetween the reduction of area due to a break in a high-pressure hydrogenenvironment and the reduction of area due to a break in the atmospherewas 90% or higher were determined to be good.

Table 4 shows the presence/absence of weld defects, the measurements ofthe surface excess weld metal heights, the results of theroom-temperature tensile tests and the results of the low-strain-ratetensile tests for the welded joints.

TABLE 4 Excess weld Low-strain- Test Weld metall height Tensile ratetensile character P1 P2 P3 defect [mm] test test J1 35 −1.3  0.53 ◯ 1.2⊚ ◯ J2 35 −1.3  0.53 ◯ 0.15 Δ ◯ J3 35 −1.3  0.53 ◯ 0.9 ⊚ ◯ J4 35 −1.3 0.53 ◯ 1.5 ⊚ ◯ J5 35 −1.31 0.51 ◯ 1.3 ⊚ ◯ J6 35 −1.31 0.51 ◯ 0.12 Δ ◯ J735 −1.31 0.51 ◯ 0.78 ⊚ ◯ J8 35 −1.31 0.51 ◯ 1.51 ⊚ ◯ J9 35 −1.35 0.44 ◯1.18 ⊚ ◯ J10 35 −1.35 0.44 ◯ 0.12 Δ ◯ J11 35 −1.35 0.44 ◯ 0.92 ⊚ ◯ J1235 −1.35 0.44 ◯ 1.5 ⊚ ◯ J13 35 −1.3  0.53 ◯ 1.15 ⊚ ◯ J14 35 −1.3  0.53 ◯0.5 Δ ◯ J15 35 −1.3  0.53 ◯ 1.48 ⊚ ◯ J16 40 −1.18 0.76 ◯ 1.22 ⊚ ◯ J17 40−1.18 0.76 ◯ 0.57 Δ ◯ J18 40 −1.18 0.76 ◯ 1.45 ⊚ ◯ J19   23 * −1.08 0.95◯ 1.08 ⊚ X J20   23 * −1.08 0.95 ◯ 0.9 Δ X J21   23 * −1.08 0.95 ◯ 1.49⊚ X J22 32 −1.04 1.02 ◯ 1.17 ⊚ ◯ J23 32 −1.04 1.02 ◯ 0.6 Δ ◯ J24 32−1.04 1.02 ◯ 1.5 ⊚ ◯ J25 31 −1.27 0.59 X 1.11 X X J26 30 −1.21 0.70 X1.01 X X J27 30 −1.21 0.70 X 0.55 X X J28   28 * −1.43 0.28 ◯ 0.29 ◯ XJ29   28 * −1.43 0.28 ◯ 0.65 ⊚ X J30   28 * −1.43 0.28 ◯ 1.65 ⊚ X J31 39  −0.96 * 1.18 ◯ 0.56 X ◯ J32 39   −0.96 * 1.18 ◯ 1.53 X ◯ J33 37  −0.86 * 1.37 ◯ 0.88 X ◯ J34 37   −0.86 * 1.37 ◯ 1.45 X ◯ J35 35 −1.350.44 ◯ 0.56 ⊚ ◯ J36 35 −1.3  0.53 ◯ 0.95 ⊚ ◯ J37 35 −1.3  0.53 ◯ 0.66 ⊚◯ J38 30 −1.21 0.70 X 0.97 X X J39 30 −1.21 0.70 X 0.54 X X * indicatesdeviation from the condition required by the invention

The column of “P1” in Table 4 lists the values of the left side ofEquation (1) obtained when the chemical compositions of the weldingmaterials of the welded joints were substituted into Equation (1). Thecolumn of “P2” lists the values of the left side of Equation (2)obtained when the chemical compositions of the welding materials of thewelded joints were substituted into Equation (2). The column of “P3”lists the values of the left side of Equation (3) obtained when thechemical compositions of the welding materials of the welded joints weresubstituted into Equation (3).

The column of “Weld defect” in Table 4 indicates the presence or absenceof weld defects. “◯” indicates that there were no weld defects. “x”indicates that there were blowholes.

The column of “Excess weld metal height” in Table 4 lists the surfaceexcess weld metal heights (mm) of the welded joints.

The column of “Tensile test” indicates the results of tensile tests. “⊚”indicates that, in each of the tensile tests, both test specimens brokein the base material or broke in the heat-welded portion (i.e. HAZbreak). “◯” indicates that the tensile strength was 800 MPa or higherbut one of the two test specimens broke in the base material and theother one broke in the weld metal. “Δ” indicates that the tensilestrength was 800 MPa or higher but both test specimens broke in the weldmetal. “x” indicates that the test specimens broke in the weld metal andthe tensile strength was lower than 800 MPa.

The column of “Low-Strain-Rate Tensile Test” lists the results of thelow-strain-rate tensile tests. “◯” indicates that the ratio between thereduction of area due to a break in the high-pressure hydrogenenvironment and the reduction of area due to a break in the atmospherewas 90% or higher. “x” indicates that the ratio between the reduction ofarea due to a break in the high-pressure hydrogen environment and thereduction of area due to a break in the atmosphere was lower than 90%.

The welded joints with test characters J1 to J18, J22 to J24 and J35 toJ37 were within the ranges of the present invention. More specifically,in each of these welded joints, the chemical compositions of the basematerial and welding material were within the ranges of the presentinvention, the chemical compositions of the base material and weldingmaterial satisfied Equation (1) and the chemical composition of thewelding material satisfied Equation (2). As a result, each of thesewelded joints had a tensile strength not lower than 800 MPa and wasfound to be good in the low-strain-rate tensile test.

Further, these welded joints had no weld defects. Especially the weldedjoints with test characters J1, J5, J9, J13, J16, J22 and J36 had noweld defects even though the welding heat inputs were relatively high.The welded joints with test characters J35 to J37 had no weld defectseven though the welding direction was vertical.

In addition, in each of the welded joints with test characters J1, J3 toJ5, J7 to J9, J11 to J13, J15, J16, J18, J22, J24 and J35 to J37, thesurface excess weld metal height h satisfied Equation (3). In otherwords, in each of these welded joints, the surface excess weld metalheight h had a value of P3 or higher. As a result, these welded jointshad particularly high tensile strengths. More specifically, these weldedjoints broke in the base material or experienced a HAZ break in thetensile test.

In each of the welded joints with test characters J19 to J21, thechemical compositions of the base material and welding material were inthe ranges of the present invention, and the chemical composition of thebase material satisfied Equation (1). However, in these welded joints,the chemical composition of the welding material did not satisfyEquation (1). As a result, these welded joints were not found to be goodin the low-strain-rate tensile tests.

In each the weld joints with test characters J25 to J27, J38 and J39,the N content in the welding material (character V or W) was too high.As a result, the welded portion had weld defects, more particularlyblowholes, resulting in a joint that was not good. As a result, thesewelded joints had a tensile strength of 800 MPa or lower.

In each of the welded joints with test characters J28 to J30, the Sicontent and Cr content in the welding material (character X) was toohigh. Further, in each of these welded joints, the chemical compositionof the welding material did not satisfy Equation (1). As a result, thesewelded joints were not found to be good in the low-strain-rate tensiletests.

In each of the welded joints with test characters J31 to J34, thechemical composition of the welding material (character Y or Z) did notsatisfy Equation (2) even though the contents of the elements werewithin the ranges of the present invention. As a result, these weldedjoints had a tensile strength of 800 MPa or lower.

INDUSTRIAL APPLICABILITY

The present invention can be suitably used in high-pressure gas piping,and more particularly, in welded joints for high-pressure hydrogen gaspiping.

The invention claimed is:
 1. A method of manufacturing a welded joint,comprising the steps of: preparing a base material having a chemicalcomposition of, in mass %: C: 0.005 to 0.1%; Si: up to 1.2%; Mn: 2.5 to6.5%; Ni: 8 to 15%; Cr: 19 to 25%; Mo: 0.01 to 4.5%; V: 0.01 to 0.5%;Nb: 0.01 to 0.5%; Al: less than 0.05%; N: 0.15 to 0.45%; O: up to 0.02%;P: up to 0.05%; and S: up to 0.04%, and a balance being iron andimpurities, preparing a welding material having a chemical compositionconsisting of, in mass %: C: 0.005 to 0.1%; Si: up to 0.7%; Mn: 0.5 to3%; Ni: 8 to 23%; Cr: 17 to 25%; Mo: 0.01 to 4%; V: 0 to 0.5%; Nb: 0 to0.5%; Al: less than 0.05%; N: less than 0.15%; O: up to 0.02%; P: up to0.03%; and s: up to 0.02%, and a balance being iron and impurities, andwelding the base material using the welding material, wherein thechemical composition of the base material satisfies Equation (1), andthe chemical composition of the welding material satisfies Equations (1)and (2),Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C≥29  (1), and0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo≤−1.0  (2), where thecontents of elements (in mass %) are substituted for the symbols ofthese elements in Equations (1) and (2).
 2. The method of manufacturinga welded joint according to claim 1, wherein the step of weldingincludes welding the base material using the welding material such thatthe welded joint has a surface excess weld metal height h (mm) thatsatisfies Equation (3),1.9×(0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo)+3≤h  (3), where thecontents of the elements in the welding material (in mass %) aresubstituted for the symbols of these elements in Equation (3).
 3. Themethod of manufacturing a welded joint according to claim 1, wherein forEquation (2), 0.31C+0.048Si−0.02Mn−0.056Cr+0.007Ni−0.013Mo of is lessthan or equal to −1.3.
 4. The method of manufacturing a welded jointaccording to claim 1, wherein the Cr content of the welding material is17 to 24.6 mass %.
 5. The method of manufacturing a welded jointaccording to claim 1, wherein the N content of the welding material isnot higher than 0.13 mass %.